Gaesatae armour value isn't so high that the AP attribute as such would make great difference (5), but as you're not going to find much in the way of good archers in the West slingers (whose missile attack tends to rival or exceed the archers' there, nevermind now with the AP effect thrown in) and javelins are your best bet.

Incidentally, the very high attack value of those German throwing-spears ought to make them pretty good for nailing most Celtic infantry. Simple math really; the mail-clad guys aside around the highest armour you're going to find in Gaul is around 5 (plus shield, but that isn't affected by AP). About half of that off with AP is 2-3 - and Roman pila and Iberian solifera don't have terribly high base attack values, 4-5 in most cases, and the slingers even lower. The German frameas tend to have something like 8 though; they ought to perform about as equally against the lightly armoured types, get past shield scores better, and ought to be right murder to unarmoured troops.